Special thanks to Whynne again as well as Geoff from Mother's Basement for helping me work through all the bits and pieces of this huge series of three theories!
Also, special thanks to itsmeveronica for usage of their custom Ralsei crying animation ►►1:07

The biggest mystery of Deltarune is how it connects to Undertale. Is it a prequel? A sequel? Something completely different? Today Loyal Theorists, I am here to help you solve this mystery. What is Deltarune? Oh boy, do I have an answer for you!